Technology for detecting a distance to an object and controlling the amount of draw-out of an imaging lens has been proposed, for example, in Japanese Published Unexamined Patent Application 51-83524 based on U.S. patent application Ser. No. 529,573, and U.S. Pat. No. 4,264,810.
In the prior art, a photo-diode was used as a photo-sensor and a photo-current from the photo-diode was processed to produce an optical image signal for the automatic focusing operation. Since the photo-current is continuously produced, the optical image signal is also continuously produced so that the optical image signal can be used to directly servo-control the amount of draw-out of the lens.
On the other hand, as the semiconductor technology has been developed, a charge store type photo-sensor such as CCD has been put into practical use. Because of an advantage of a smaller photo-sensitive area than the photo-diode, the CCD has been noted as the photo-sensor of the automatic focusing device. However, since an output of the CCD photo-sensor is produced at every integration period, the servo-output for driving the lens is also time-serial and intermittent. In addition, since an automatic gain control (AGC) is usually carried out for the integration period in order to control the output of the CCD photo-sensor to an appropriate output level, a time interval of the servo-outputs significantly varies with the light quantity. Consequently, when the servo system is designed to respond to all of the serially generated servo-outputs, the servo system may oscillate and the lens may hunt.